Peach's fair has greater reach, but less range.
This is a custom terminology that as far as I know, I'm the only one to really use, so I'll elaborate.
By reach, I mean that the distance from Peach's fair hitbox to her center of gravity is greater than that of Marth's fair to his. By range, I mean that there is less distance between the edge of her fair's hitbox and the edge of the closest hurtbox to her, compared to Marth's - essentially degree of disjoint.
What does this mean? Well, it basically means that you can both punish each other for ever barely missing a fair, and that Peach can hit a little farther away from her than one might think.